The invention relates generally to cryogenic fluid dispensing systems, and, more particularly, to a manifold system for cryogenic fluid dispensing systems that use multiple liquid cylinders as the source of cryogenic fluid.
Cryogenic liquids, that is, liquids having a boiling point generally below xe2x88x92150xc2x0 F. at atmospheric pressure, are used in a variety of applications. Many of these applications require that the cryogen be supplied as a high pressure gas. For example, high pressure nitrogen and argon gases are required for laser welding while high pressure nitrogen, oxygen and argon gases are required for laser cutting. Such cryogens are typically stored as liquids, however, because one volume of liquid produces many volumes of gas (600-900 volumes of gas per one volume of liquid) when the liquid is permitted to vaporize/boil and warm to ambient temperature. To store an equivalent amount of gas requires that the gas be stored at very high pressure. This would require heavier and larger tanks and expensive pumps or compressors.
Industrial applications such as laser welding and cutting require that the cryogenic gases be provided at pressures in the range of approximately 400-420 psi and flow rates in the range of approximately 1500-2500 SCFH. It is known in the prior art that such high pressures and flow rates may be obtained by connecting a number of cryogenic liquid storage tanks or cylinders together in parallel to form a xe2x80x9cbankxe2x80x9d of liquid cylinders. A prior art bank of cryogenic liquid cylinders is illustrated in FIG. 1.
As illustrated in FIG. 1, the bank of cylinders features a manifold, indicated in general at 10, which is connected to insulated cryogenic liquid cylinders 12a-12d. More specifically, the manifold 10 includes a liquid header 14 that is connected to the dip tubes 16a-16d of cylinders 12a-12d via flexible lines 18a-18d. Similarly, the head spaces of cylinders 12a-12b are connected to a gas header 22 of manifold 10 by flexible lines 24a-24d. Liquid is forced out of the cylinders through their dip tubes due to the internal pressurization that occurs when the liquid within the cylinders vaporizes as it is warmed over time.
The bank of cylinders provides cryogenic liquid to a use point, typically including a vaporizer, through liquid header 14 and port 26. Gas header 22 equalizes the pressures within the cylinders. An economizer circuit 28 permits gas to be withdrawn directly from the head spaces of the cylinders and delivered to the use point when the pressure within the gas header exceeds a predetermined level. As a result, venting of cryogenic vapor is avoided. Greater flow rates at high pressures may be obtained by adding additional manifold sections and cylinders to the bank via fittings 20a and 20b. 
Situations may occur, however, where liquid is withdrawn from one of the cylinders faster than the others. In such situations, one cylinder may empty of liquid prior to the other cylinders. Prior art manifolds encounter difficulties in handling such occurrences. More specifically, if liquid cylinder 12b empties of liquid prior to the other cylinders, as illustrated in FIG. 1, gas from cylinder 12b will quickly travel out of dip tube 16b, as illustrated by arrows 32, through liquid header 14 and out of port 26. As a result, the pressure within the bank will collapse as gas travels from the individual cylinders 12a, 12c and 12d into the gas header 22, as illustrated by arrows 34, 36 and 38, and into cylinder 12b, as illustrated by arrows 42. In other words, the vapor from cylinders 12a, 12c and 12d and gas header 22 escapes through the path of least resistance through the empty cylinder 12b, dip tube 16b, liquid header 14 and port 26. When the pressure within the bank of cylinders collapses, the system stops delivering high pressure cryogenic fluid and the operation (such as welding or cutting) is interrupted. It is therefore desirable to provide a manifold that prevents interruptions in the delivery of high pressure cryogenic fluid from a bank of liquid cylinders when the liquid supply in a cylinder is exhausted prior to the other cylinders in the bank.
Furthermore, if the liquid level in one cylinder drops below the liquid level of the other cylinders, system efficiency suffers. That is, when the liquid level within a cylinder becomes low, its internal pressure also drops. As a result, the pressure within the remaining cylinders also drops as vapor from the gas header 22 travels into the cylinder with the low liquid level. The bank of FIG. 1 therefore requires many cylinders to supply cryogenic fluid at an acceptable pressure and flow rate. In addition, if the pressure within one cylinder drops, liquid in the liquid header 14 may back flow into the low pressure cylinder so that fluid delivery is interrupted. It is therefore desirable to provide a manifold that withdraws liquidly evenly from a number of cylinders and prevents the back flow of liquid into the cylinders.
Prior art systems often combine two banks of cylinders of the type illustrated in FIG. 1. One bank is designated the xe2x80x9cprimaryxe2x80x9d or xe2x80x9cservicexe2x80x9d bank while the other bank is designated the xe2x80x9csecondaryxe2x80x9d or xe2x80x9creservexe2x80x9d bank. These are coupled through an electronic control system which typically flows the primary bank and, when it is exhausted of liquid, changes over automatically to the secondary bank and simultaneously activates an alarm. The electronic control system also typically provides a means by which the function of the two banks may be reversed after the empty cylinders of the primary bank are replaced.
An example of such a manifold control system may be found in U.S. Pat. No. 5,062,443 to Maric. The control system of the Maric ""443 patent features first and second conduits that connect with primary and secondary cryogenic fluid supply sources (such as liquid cylinder banks). Each conduit includes both a pressure sensor that senses fluid flow pressure and a solenoid-operated on-off fluid flow control valve positioned downstream of the pressure sensor. An electrical circuit is in communication with the pressure sensors and controls the operation of the valves to switch them on and off so as to permit or prevent fluid flow through the respective conduit.
With both fluid sources available, the electrical circuit of the Maric ""443 patent only permits one of the fluid sources to provide fluid flow at one time. When the first fluid source delivers fluid at a pressure below a predetermined minimum value, as detected by the first pressure sensor, the electrical circuit generates a signal to close the solenoid valve in the first conduit and simultaneously opens the solenoid valve in the second conduit so that the fluid flow then commences therein. The exhausted first fluid supply may then be replaced. The electrical circuit will switch back to the first fluid supply and conduit when the pressure within the second conduit drops below the predetermined minimum value. If the exhausted first fluid supply is not replaced and the second fluid supply becomes exhausted, the electrical circuit closes the solenoid valve of the second conduit, the solenoid valve of the first conduit remains closed and an alarm is activated. The system remains on standby until one or more of the fluid supplies is replaced.
While the control system of the Maric ""443 patent is effective, the pressure within a conduit may fall below the predetermined minimum value while liquid remains in the corresponding fluid supply. As a result, the system may change over to the other conduit and fluid supply while liquid still exists in the original fluid supply. It is therefore desirable to provide a manifold control system that prevents residual liquid in the original primary or service liquid cylinder bank after change over to the secondary or reserve liquid cylinder bank.
Accordingly, it is an object of the present invention to provide a manifold that prevents interruptions in the delivery of high pressure cryogenic fluid from a bank of liquid cylinders when the liquid supply in a cylinder is exhausted prior to the other cylinders in the bank.
It is another object of the present invention to provide a manifold that withdraws liquidly evenly from a number of cylinders.
It is another object of the present invention to provide a manifold that prevents back flow of liquid into a cylinder of a cylinder bank.
It is still another object of the present invention to provide a manifold control system that prevents residual liquid in the original primary or service liquid cylinder bank after change over to the secondary or reserve liquid cylinder bank.
It is still another object of the present invention to provide a manifold through which simple and efficient change out of cylinders may be accomplished.
The present invention is directed a system for selectively dispensing cryogenic liquid from a primary bank manifold that is in communication with a first plurality of cylinders and a secondary bank manifold that is in communication with a second plurality of cylinders. The primary bank and secondary bank manifolds each include a gas header and a liquid header. The first plurality of cylinders communicate with the gas header of the primary bank manifold through excess flow valves and the liquid header through spring-loaded check valves. Similarly, the second plurality of cylinders communicate with the gas header and liquid header of the secondary bank manifold via excess flow and spring-loaded check valves, respectively. As a result, liquid is withdrawn from either the first or second plurality of tanks in a generally even fashion. In addition, sudden drops of manifold pressure when one of the cylinders in the first or second plurality of cylinders goes empty is avoided.
Selection between the dispensing of cryogenic liquid from either the primary bank manifold or the secondary bank manifold is controlled by an automated control system. The control system includes valves that control the flow of cryogenic liquid from the primary bank manifold and the secondary bank manifold to a use point line. Pressure gauges detect the pressure of the cryogenic liquid flowing from either one of the manifolds. The valves and pressure gauges communicate with a controller. If the pressure gauge corresponding to the manifold that is dispensing detects a drop in pressure to a level below a predetermined pressure, the controller closes the valve for the dispensing manifold and opens the valve for the other, originally idle, manifold. As a result, dispensing commences from the manifold that originally was sitting idle. The controller checks the pressure in the manifold that was originally dispensing to determine if the pressure therein has once again risen to a level above the predetermined pressure due to remaining residual liquid. If the pressure has once again risen to an acceptable level, the controller reconfigures the system to dispense from the originally dispensing manifold.
A control panel allows for manual manifold selection and indicates which manifold is selected and when there is a change in system status.
Economizer circuits in the manifolds and control system allow gas to be directed to the use point. As a result, venting of the manifold or cylinders is unnecessary.
The following detailed description of embodiments of the invention, taken in conjunction with the appended claims and accompanying drawings, provide a more complete understanding of the nature and scope of the invention.